Ras activation in platelets after stimulation of the thrombin receptor, thromboxane A2 receptor or protein kinase C

Preparing to strike: Acute events in signaling by the serpentine receptor for thromboxane A2

Over the last two decades, awareness of the (patho)physiological roles of thromboxane A₂ signaling has been greatly extended. From humble beginnings as a short-lived stimulus that activates platelets and causes vasoconstriction to a dichotomous receptor system involving multiple endogenous ligands capable of modifying tissue homeostasis and disease generation in almost every tissue of the body. Thromboxane A₂ receptor (TP) signal transduction is associated with the pathogenesis of cancer, atherosclerosis, heart disease, asthma, and host response to parasitic infection amongst others. The two receptors mediating these cellular responses (TPα and TPβ) are derived from a single gene (TBXA2R) through alternative splicing. Recently, knowledge about the mechanism(s) of signal propagation by the two receptors has undergone a revolution in understanding. Not only have the structural relationships associated with G-protein coupling been established but the modulation of that signaling by post-translational modification to the receptor has come sharply into focus. Moreover, the signaling of the receptor unrelated to G-protein coupling has become a burgeoning field of endeavor with over 70 interacting proteins currently identified. These data are reshaping the concept of TP signaling from a mere guanine nucleotide exchange factors for Gα activation to a nexus for the convergence of diverse and poorly characterized signaling pathways. This review summarizes the advances in understanding in TP signaling, and the potential for new growth in a field that after almost 50 years is finally coming of age.

Characterisation of the Ral GTPase inhibitor RBC8 in human and mouse platelets

The Ral GTPases, RalA and RalB, have been implicated in numerous cellular processes, but are most widely known for having regulatory roles in exocytosis. Recently, we demonstrated that deletion of both Ral genes in a platelet-specific mouse gene knockout caused a substantial defect in surface exposure of P-selectin, with only a relatively weak defect in platelet dense granule secretion that did not alter platelet functional responses such as aggregation or thrombus formation. We sought to investigate the function of Rals in human platelets using the recently described Ral inhibitor, RBC8. Initial studies in human platelets confirmed that RBC8 could effectively inhibit Ral GTPase activation, with an IC50 of 2.2 μM and 2.3 μM for RalA and RalB, respectively. Functional studies using RBC8 revealed significant, dose-dependent inhibition of platelet aggregation, secretion (α- and dense granule), integrin activation and thrombus formation, while α-granule release of platelet factor 4, Ca2+ signalling or phosphatidylserine exposure were unaltered. Subsequent studies in RalAB-null mouse platelets pretreated with RBC8 showed dose-dependent decreases in integrin activation and dense granule secretion, with significant inhibition of platelet aggregation and P-selectin exposure at 10 μM RBC8. This study strongly suggests therefore that although RBC8 is useful as a Ral inhibitor in platelets, it is likely also to have off-target effects in the same concentration range as for Ral inhibition. So, whilst clearly useful as a Ral inhibitor, interpretation of data needs to take this into account when assessing roles for Rals using RBC8.

Small GTPases in platelet membrane trafficking

Our understanding of fundamental biological processes within platelets is continually evolving. A critical feature of platelet biology relates to the intricate uptake, packaging and release of bioactive cargo from storage vesicles, essential in mediating a range of classical (haemostasis/thrombosis) and non-classical (regeneration/inflammation/metastasis) roles platelets assume. Pivotal to the molecular control of these vesicle trafficking events are the small GTPases of the Ras superfamily, which function as spatially distinct, molecular switches controlling essential cellular processes. Herein, we specifically focus on members of the Rab, Arf and Ras subfamilies, which comprise over 130 members and platelet proteomic datasets suggest that more than half of these are expressed in human platelets. We provide an update of current literature relating to trafficking roles for these GTPases in platelets, particularly regarding endocytic and exocytic events, but also vesicle biogenesis and provide speculative argument for roles that other related GTPases and regulatory proteins may adopt in platelets. Advances in our understanding of small GTPase function in the anucleate platelet has been hampered by the lack of specific molecular tools, but it is anticipated that this will be greatly accelerated in the years ahead and will be crucial to the identification of novel therapeutic targets controlling different platelet processes.

TC21/RRas2 regulates glycoprotein VI-FcRγ-mediated platelet activation and thrombus stability

Essentials RAS proteins are expressed in platelets but their functions are largely uncharacterized. TC21/RRas2 is required for glycoprotein VI-induced platelet responses and for thrombus stability in vivo. TC21 regulates platelet aggregation by control of αIIb β3 integrin activation, via crosstalk with Rap1b. This is the first indication of functional importance of a proto-oncogenic RAS protein in platelets.

SUMMARY

Background Many RAS family small GTPases are expressed in platelets, including RAC, RHOA, RAP, and HRAS/NRAS/RRAS1, but most of their signaling and cellular functions remain poorly understood. Like RRAS1, TC21/RRAS2 reverses HRAS-induced suppression of integrin activation in CHO cells. However, a role for TC21 in platelets has not been explored. Objectives To determine TC21 expression in platelets, TC21 activation in response to platelet agonists, and roles of TC21 in platelet function in in vitro and in vivo thrombosis. Results We demonstrate that TC21 is expressed in human and murine platelets, and is activated in response to agonists for the glycoprotein (GP) VI-FcRγ immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor, in an Src-dependent manner. GPVI-induced platelet aggregation, integrin αIIb β3 activation, and α-granule and dense granule secretion, as well as phosphorylation of Syk, phospholipase Cγ2, AKT, and extracellular signal-regulated kinase, were inhibited in TC21-deficient platelets ex vivo. In contrast, these responses were normal in TC21-deficient platelets following stimulation with P2Y, protease-activated receptor 4 and C-type lectin receptor 2 receptor agonists, indicating that the function of TC21 in platelets is GPVI-FcRγ-ITAM-specific. TC21 was required for GPVI-induced activation of Rap1b. TC21-deficient mice did not show a significant delay in injury-induced thrombosis as compared with wild-type controls; however, thrombi were unstable. Hemostatic responses showed similar effects. Conclusions TC21 is essential for GPVI-FcRγ-mediated platelet activation and for thrombus stability in vivo via control of Rap1b and integrins.

RAP GTPases and platelet integrin signaling

Platelets are highly specialized cells that continuously patrol the vasculature to ensure its integrity (hemostasis). At sites of vascular injury, they are able to respond to trace amounts of agonists and to rapidly transition from an anti-adhesive/patrolling to an adhesive state (integrin inside-out activation) required for hemostatic plug formation. Pathological conditions that disturb the balance in the underlying signaling processes can lead to unwanted platelet activation (thrombosis) or to an increased bleeding risk. The small GTPases of the RAP subfamily, highly expressed in platelets, are critical regulators of cell adhesion, cytoskeleton remodeling, and MAP kinase signaling. Studies by our group and others demonstrate that RAP GTPases, in particular RAP1A and RAP1B, are the key molecular switches that turn on platelet activation/adhesiveness at sites of injury. In this review, we will summarize major findings on the role of RAP GTPases in platelet biology with a focus on the signaling pathways leading to the conversion of integrins to a high-affinity state.

The Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) Binder Rasa3 Regulates Phosphoinositide 3-kinase (PI3K)-dependent Integrin αIIbβ3 Outside-in Signaling

The class I PI3K family of lipid kinases plays an important role in integrin α_IIbβ₃ function, thereby supporting thrombus growth and consolidation. Here, we identify Ras/Rap1GAP Rasa3 (GAP1^IP4BP) as a major phosphatidylinositol 3,4,5-trisphosphate-binding protein in human platelets and a key regulator of integrin α_IIbβ₃ outside-in signaling. We demonstrate that cytosolic Rasa3 translocates to the plasma membrane in a PI3K-dependent manner upon activation of human platelets. Expression of wild-type Rasa3 in integrin α_IIbβ₃-expressing CHO cells blocked Rap1 activity and integrin α_IIbβ₃-mediated spreading on fibrinogen. In contrast, Rap1GAP-deficient (P489V) and Ras/Rap1GAP-deficient (R371Q) Rasa3 had no effect. We furthermore show that two Rasa3 mutants (H794L and G125V), which are expressed in different mouse models of thrombocytopenia, lack both Ras and Rap1GAP activity and do not affect integrin α_IIbβ₃-mediated spreading of CHO cells on fibrinogen. Platelets from thrombocytopenic mice expressing GAP-deficient Rasa3 (H794L) show increased spreading on fibrinogen, which in contrast to wild-type platelets is insensitive to PI3K inhibitors. Together, these results support an important role for Rasa3 in PI3K-dependent integrin α_IIbβ₃-mediated outside-in signaling and cell spreading.

[Anti-platelets without a bleeding risk: novel targets and strategies]

Anti-platelet agents such as aspirin, clopidogrel and antagonists of integrin αIIbβ3 allowed to efficiently reduce morbidity and mortality associated with arterial thrombosis. A major limit of these drugs is that they increase the risk of bleeding. During the last few years, several innovative anti-thrombotic strategies with a potentially low bleeding risk were proposed. These approaches target the collagen receptor glycoprotein (GP) VI, the GPIb/von Willebrand factor axis, the thrombin receptor PAR-1, the activated form of integrin αIIbβ3 or the ADP receptor P2Y1. While an antagonist of PAR-1 was recently marketed, the clinical proofs of the efficiency and safety of the other agents remain to be established. This review evaluates these new anti-platelet approaches toward safer anti-thrombotic therapies.

RASA3 is a critical inhibitor of RAP1-dependent platelet activation

The small GTPase RAP1 is critical for platelet activation and thrombus formation. RAP1 activity in platelets is controlled by the GEF CalDAG-GEFI and an unknown regulator that operates downstream of the adenosine diphosphate (ADP) receptor, P2Y12, a target of antithrombotic therapy. Here, we provide evidence that the GAP, RASA3, inhibits platelet activation and provides a link between P2Y12 and activation of the RAP1 signaling pathway. In mice, reduced expression of RASA3 led to premature platelet activation and markedly reduced the life span of circulating platelets. The increased platelet turnover and the resulting thrombocytopenia were reversed by concomitant deletion of the gene encoding CalDAG-GEFI. Rasa3 mutant platelets were hyperresponsive to agonist stimulation, both in vitro and in vivo. Moreover, activation of Rasa3 mutant platelets occurred independently of ADP feedback signaling and was insensitive to inhibitors of P2Y12 or PI3 kinase. Together, our results indicate that RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI/RAP1 signaling and suggest that P2Y12 signaling is required to inhibit RASA3 and enable sustained RAP1-dependent platelet activation and thrombus formation at sites of vascular injury. These findings provide insight into the antithrombotic effect of P2Y12 inhibitors and may lead to improved diagnosis and treatment of platelet-related disorders.

Platelet function and Isoprostane biology. Should isoprostanes be the newest member of the orphan-ligand family?

While there have been many reports investigating the biological activity and signaling mechanisms of isoprostanes, their role in biology, particularly in platelets, appears to still be underestimated. Moreover, whether these lipids have their own receptors is still debated, despite multiple reports that discrete receptors for isporpstanes do exist on platelets, vascular tissues, amongst others. This paper provides a review of the important literature of isoprostanes and provides reasoning that isoprostanes should be classified as orphan ligands until their receptor(s) is/are identified.

CalDAG-GEFI and protein kinase C represent alternative pathways leading to activation of integrin alphaIIbbeta3 in platelets

Second messenger-mediated inside-out activation of integrin alphaIIbbeta3 is a key step in platelet aggregation. We recently showed strongly impaired but not absent alphaIIbbeta3-mediated aggregation of CalDAG-GEFI-deficient platelets activated with various agonists. Here we further evaluated the roles of CalDAG-GEFI and protein kinase C (PKC) for alphaIIbbeta3 activation in platelets activated with a PAR4 receptor-specific agonist, GYPGKF (PAR4p). Compared with wild-type controls, platelets treated with the PKC inhibitor Ro31-8220 or CalDAG-GEFI-deficient platelets showed a marked defect in aggregation at low (< 1mM PAR4p) but not high PAR4p concentrations. Blocking of PKC function in CalDAG-GEFI-deficient platelets, how-ever, strongly decreased aggregation at all PAR4p concentrations, demonstrating that CalDAG-GEFI and PKC represent separate, but synergizing, pathways important for alphaIIbbeta3 activation. PAR4p-induced aggregation in the absence of CalDAG-GEFI required cosignaling through the Galphai-coupled receptor for ADP, P2Y12. Independent roles for CalDAG-GEFI and PKC/Galphai signaling were also observed for PAR4p-induced activation of the small GTPase Rap1, with CalDAG-GEFI mediating the rapid but reversible activation of this small GTPase. In summary, our study identifies CalDAG-GEFI and PKC as independent pathways leading to Rap1 and alphaIIbbeta3 activation in mouse platelets activated through the PAR4 receptor.

Thromboxane A2 is a key regulator of pathogenesis during Trypanosoma cruzi infection

Chagas' disease is caused by infection with the parasite Trypanosoma cruzi. We report that infected, but not uninfected, human endothelial cells (ECs) released thromboxane A(2) (TXA(2)). Physical chromatography and liquid chromatography-tandem mass spectrometry revealed that TXA(2) is the predominant eicosanoid present in all life stages of T. cruzi. Parasite-derived TXA(2) accounts for up to 90% of the circulating levels of TXA(2) in infected wild-type mice, and perturbs host physiology. Mice in which the gene for the TXA(2) receptor (TP) has been deleted, exhibited higher mortality and more severe cardiac pathology and parasitism (fourfold) than WT mice after infection. Conversely, deletion of the TXA(2) synthase gene had no effect on survival or disease severity. TP expression on somatic cells, but not cells involved in either acquired or innate immunity, was the primary determinant of disease progression. The higher intracellular parasitism observed in TP-null ECs was ablated upon restoration of TP expression. We conclude that the host response to parasite-derived TXA(2) in T. cruzi infection is possibly an important determinant of mortality and parasitism. A deeper understanding of the role of TXA(2) may result in novel therapeutic targets for a disease with limited treatment options.

Ca2+/calmodulin binds and dissociates K-RasB from membrane

We have investigated the interaction of calmodulin (CaM) with Ras-p21 and the significance of this association. All Ras-p21 isoforms tested (H-, K-, and N-Ras) were detected in the particulate fraction of human platelets and MCF-7 cells, a human breast cancer cell line. In MCF-7 cells, H- and N-Ras were also detected in the cytosolic fraction. K-RasB from platelet and MCF-7 cell lysates was found to bind CaM in a Ca2+ -dependent but GTPgammaS-independent manner. The yeast two-hybrid analysis demonstrated that K-RasB binds to CaM in vivo. Incubation of isolated membranes from platelet and MCF-7 cells with CaM caused dissociation of only K-RasB from membranes in a Ca2+ -dependent manner. CaM antagonist, W7, inhibited dissociation of K-RasB. Addition of platelet or MCF-7 cytosol alone to isolated platelet membranes did not cause dissociation of K-RasB and only addition of exogenous CaM caused dissociation. The results suggest a potential role for Ca2+/CaM in the regulation of K-RasB function.

Oncogenic transformation induces tumor angiogenesis: a role for PAR1 activation

The formation of new blood vessels is a critical determinant of tumor progression. We find that Par1 gene expression plays a central role in blood vessel recruitment in animal models. By in vivo injection of either Matrigel plugs containing Par1-expressing cells or of rat prostatic carcinoma cells transfected with tetracycline-inducible Par1 expression vectors, we show that Par1 significantly enhances both angiogenesis and tumor growth. Several vascular endothelial growth factor (VEGF) splice forms are induced in cells expressing Par1. Activation of PAR1 markedly augments the expression of VEGF mRNAs and of functional VEGFs as determined by in vitro assays for endothelial tube alignment and bovine aortic endothelial cell proliferation. Because neutralizing anti-VEGF antibodies potently inhibited Par1-induced endothelial cell proliferation, we conclude that Par1-induced angiogenesis requires VEGF. Specific inhibitors of protein kinase C (PKC), Src, and phosphatidylinositol 3-kinase (PI3K) inhibit Par1-induced VEGF expression, suggesting the participation of these kinases in the process. We also show that oncogenic transformation by genes known to be part of PAR1 signaling machinery is sufficient to increase VEGF expression in NIH 3T3 cells. These data support the novel notion that initiation of cell signaling either by activating PAR1 or by the activated forms of oncogenes is sufficient to induce VEGF and hence angiogenesis.

Platelet ERK2 activation by thrombin is dependent on calcium and conventional protein kinases C but not Raf-1 or B-Raf

Extracellular signal-regulated kinase (ERK) activation pathways have been well characterized in a number of cell types but very few data are available for platelets. The thrombin-induced signaling pathway leading to ERK2 activation in platelets is largely uncharacterized. In this study, we investigated the kinases involved in thrombin-induced ERK2 activation in conditions of maximal ERK2 activation. We found that thrombin-induced mitogen-activated protein kinase/ERK kinase (MEK)1/2 activation was necessary for ERK2 phosphorylation. We obtained strong evidence that conventional protein kinase Cs (PKCs) and calcium are involved in thrombin-induced ERK2 activation. First, ERK2 and MEK1/2 phosphorylation was totally inhibited by low concentrations (1 microM) of RO318425, a specific inhibitor of conventional PKCs. Second, Ca(2+), from either intracellular pools or the extracellular medium, was necessary for ERK2 activation and conventional PKC activation, excluding the involvement of a new class of calcium-insensitive PKCs. Third, LY294002 and wortmannin had no significant effect on ERK2 activation, even at concentrations that inhibit phosphatidylinositol (PI)3-kinase (5 microM to 25 microM and 50 nM, respectively). This suggests that PI3-kinase was not necessary for ERK2 activation and therefore, that PI3-kinase-dependent atypical PKCs were not involved. Surprisingly, in contrast to proliferative cells, we found that the serine/threonine kinases Raf-1 and B-Raf were not an intermediate kinase between conventional PKCs and MEK1/2. After immunoprecipitation of Raf-1 and B-Raf, the basal glutathione S-transferase-MEK1 phosphorylation observed in resting platelets was not upregulated by thrombin and was still observed in the absence of anti-Raf-1 or anti-B-Raf antibodies. In these conditions, the in vitro cascade kinase assay did not detect any MEK activity. Thus in platelets, thrombin-induced ERK2 activation is activated by conventional PKCs independently of Raf-1 and B-Raf activation.

Regulation of extracellular signal-regulated kinase cascades by alpha- and beta-isoforms of the human thromboxane A(2) receptor

Thromboxane A(2) (TXA(2)) stimulates mitogenic growth of vascular smooth muscle. In humans, TXA(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta. To investigate the mechanism of TXA(2)-mediated mitogenesis, regulation of extracellular signal-regulated kinase (ERK) signaling was examined in human embryonic kidney 293 cells stably overexpressing the individual TP isoforms. The TXA(2) mimetic 9,11-dideoxy-9alpha,11alpha-methano epoxy prostaglandin F(2alpha) (U46619) elicited concentration- and time-dependent activation of ERK1 and -2 through both TPs with maximal TPalpha- and TPbeta-mediated ERK activation observed after 10 and 5 min, respectively. U46619-mediated ERK activation was inhibited by the TP antagonist [1S-[1alpha,2beta-(5Z)-3beta,4alpha-]]-7-[3-[[2-(phenylamino)carbonyl]hydrazine] methyl]-7-oxabicyclo[-2,2,1-]hept-2yl]-5-heptenoic acid (SQ29,548), and by the mitogen-activated protein kinase kinase inhibitor 2'-amino-3'-methoxyflavone (PD 98059). Although ERK activation through TPalpha was dependent on 2-[1-(dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF 109203X)-sensitive protein kinase (PK) Cs, ERK activation through TPbeta was only partially dependent on PKCs. ERK activation through both TPalpha and TPbeta was dependent on PKA and phosphoinositide 3-kinase (PI3K) class 1(A), but not class 1(B), and was modulated by Harvey-Ras, A-Raf, c-Raf, and Rap1B/B-Raf and also involved transactivation of the epidermal growth factor receptor. Additionally, PKB/Akt was activated through TPalpha and TPbeta in a PI3K-dependent manner. In conclusion, we have defined the key components of TXA(2)-mediated ERK signaling and have established that both TPalpha and TPbeta are involved. TXA(2)-mediated ERK activation through the TPs is a complex event involving PKC-, PKA-, and PI3K-dependent mechanisms in addition to transactivation of the EGF receptor. TPalpha and TPbeta mediate ERK activation through similar mechanisms, although the time frame for maximal ERK activation and PKC dependence differs.

Regulation of RAS in human platelets. Evidence that activation of RAS is not sufficient to lead to ERK1-2 phosphorylation

In this study, we show that the G protein-coupled receptor agonist thrombin, the glycoprotein VI agonist convulxin, and the cytokine receptor Mpl agonist thrombopoietin (TPO) are able to induce activation of RAS in human platelets. Recruitment of GRB2 by tyrosine-phosphorylated proteins in response to TPO and convulxin but not by thrombin occurred with a similar time-course to RAS activation, consistent with a causal relationship. On the other hand, activation of ERK2 by thrombin and convulxin is delayed and also inhibited by the protein kinase C inhibitor Ro-31 8220, whereas RAS activation is unaffected. Further evidence for differential regulation of RAS and ERK is provided by the observations that TPO, which activates RAS but not protein kinase C, does not activate ERK, and that the inhibitor of SRC kinases PP1 inhibits activation of RAS but not ERK2 in response to thrombin. Our results demonstrate that activation of RAS is not necessarily coupled to ERK in human platelets.

Identification of shc as the primary protein binding to the tyrosine-phosphorylated beta 3 subunit of alpha IIbbeta 3 during outside-in integrin platelet signaling

Outside-in signaling mediated by the integrin alpha(IIb)beta(3) (GPIIbIIIa) is critical to platelet function and has been shown to involve the phosphorylation of tyrosine residues on the cytoplasmic tail of beta(3). To identify proteins that bind directly to phosphorylated beta(3), we utilized an affinity column consisting of a peptide modeled on the tyrosine-phosphorylated cytoplasmic domain of beta(3). Tandem mass spectrometric sequencing and immunoblotting demonstrated that Shc was the primary protein binding to phosphorylated beta(3). To determine the involvement of Shc in outside-in alpha(IIb)beta(3) signaling, the phosphorylation of Shc during platelet aggregation was examined; transient Shc phosphorylation was observed when thrombin-stimulated platelets were allowed to aggregate or when aggregation was induced by an LIBS (ligand-induced binding site) antibody, D3. Moreover, Shc was co-immunoprecipitated with tyrosine-phosphorylated beta(3) in detergent lysates of aggregated platelets. Using purified, recombinant protein, it was found that the binding of Shc to monophosphorylated (C-terminal tyrosine) and diphosphorylated beta(3) peptides was direct, demonstrating Shc recognition motifs on phospho-beta(3). Aggregation-induced Shc phosphorylation was also observed to be robust in platelets from wild-type mice, but not in those from mice expressing (Y747F,Y759F) beta(3), which are defective in outside-in alpha(IIb)beta(3) signaling. Thus, Shc is the primary downstream signaling partner of beta(3) in its tyrosine phosphorylation outside-in signaling pathway.

Low-density lipoprotein activates the small GTPases Rap1 and Ral in human platelets

Physiological concentrations of low-density lipoprotein (LDL) sensitize blood platelets to alpha-thrombin- and collagen-induced secretion, and after prolonged contact trigger secretion independent of other agonists. Here we report that LDL activates the small GTPases Rap1 and Ral but not Ras, as assessed by specific precipitation of the GTP-bound enzymes. In unstirred suspensions, the inhibitor SB203580 blocks Rap1 activation by 60-70%, suggesting activation via p38 mitogen-activated protein kinase and a second, unidentified route. Inhibitors of cyclooxygenase (indomethacin) and the thromboxane A(2) (TxA(2)) receptor (SQ30741) induce complete inhibition, indicating that Rap1 activation is the result of TxA(2) formation. Stirring reveals a second, TxA(2)-independent Rap1 activation, which correlates quantitatively with a slow induction of dense granule secretion. Both pathways are unaffected by inhibitors of ligand binding to integrin alpha(IIb)beta(3). The results suggest that Rap1 and Ral, but not Ras, may take part in signalling routes initiated by LDL that initially enhance the sensitivity of platelets to other agonists and later trigger LDL-dependent secretion.

Farnesylcysteine analogues inhibit store-regulated Ca2+ entry in human platelets: evidence for involvement of small GTP-binding proteins and actin cytoskeleton

We have investigated the mechanism of Ca(2+) entry into fura-2-loaded human platelets by preventing the prenylation of proteins such as small GTP-binding proteins. The farnesylcysteine analogues farnesylthioacetic acid (FTA) and N-acetyl-S-geranylgeranyl-L-cysteine (AGGC), which are inhibitors of the methylation of prenylated and geranylgeranylated proteins respectively, significantly decreased thrombin-evoked increases in intracellular free Ca(2+) concentration ([Ca(2+)](i)) in the presence, but not in the absence, of external Ca(2+), suggesting a relatively selective inhibition of Ca(2+) entry over internal release. Both these compounds and N-acetyl-S-farnesyl-L-cysteine, which had similar effects to those of FTA, also decreased Ca(2+) entry evoked by the depletion of intracellular Ca(2+) stores with thapsigargin. The inactive control N-acetyl-S-geranyl-L-cysteine was without effect. Patulin, an inhibitor of prenylation that is inert with respect to methyltransferases, also decreased store-regulated Ca(2+) entry. Cytochalasin D, an inhibitor of actin polymerization, significantly decreased store-regulated Ca(2+) entry in a time-dependent manner. Both cytochalasin D and the farnesylcysteine analogues FTA and AGGC inhibited actin polymerization; however, when evoking the same extent of decrease in actin filament formation, FTA and AGGC showed greater inhibitory effects on Ca(2+) entry, indicating a cytoskeleton-independent component in the regulation of Ca(2+) entry by small GTP-binding-protein. These findings suggest that prenylated proteins such as small GTP-binding proteins are involved in store-regulated Ca(2+) entry through actin cytoskeleton-dependent and cytoskeleton-independent mechanisms in human platelets.

DDM-PGE(2)-mediated cytoprotection in renal epithelial cells by a thromboxane A(2) receptor coupled to NF-kappaB

The present studies were conducted to determine the pharmacological nature of a cytoprotective 11-deoxy-16,16-dimethyl-PGE(2) (DDM-PGE(2)) receptor in LLC-PK(1) cells. DDM-PGE(2)-mediated cytoprotection against 2,3,5-(trisglutathion-S-yl)hydroquinone (TGHQ)-mediated cytotoxicity can be reproduced using thromboxane A(2) (TXA(2)) receptor (TP) agonists (U46619 and IBOP), and the cytoprotective response to DDM-PGE(2) and TP agonists is inhibited by TP antagonists (SQ-29,548 and ISAP). Western blot analysis using an antipeptide antibody against the human platelet TP receptor (55 kDa) identified a particulate associated 54-kDa protein. DDM-PGE(2)-mediated 12-O-tetradecanoyl phorbol-13-acetate (TPA) responsive element (TRE) binding activity is not inhibited by cyclooxygenase inhibitors (aspirin and indomethacin) or a TXA(2) synthase inhibitor (sulfasalazine), suggesting that the biological response to DDM-PGE(2) is not dependent on de novo TXA(2) biosynthesis. Peak DDM-PGE(2)- and U46619-mediated TRE binding activity and nuclear factor-kappaB (NF-kappaB) binding activity are inhibited by SQ-29,548. The full cytoprotective response to DDM-PGE(2) requires an 8-h pulse with agonist. DDM-PGE(2)-mediated TRE and NF-kappaB binding activity remain elevated in the presence of agonist and rapidly decay following agonist washout, suggesting a direct correlation between DDM-PGE(2)-mediated cytoprotection and persistent DNA binding activities. TPA, a protein kinase C activator, induces cytoprotection and a persistent increase of NF-kappaB binding activity. DDM-PGE(2)-mediated cytoprotection and NF-kappaB binding activity but not TRE binding activity are inhibited by sulfasalazine. We conclude that the DDM-PGE(2) receptor is a TP receptor and that the cytoprotective response may be mediated in part by NF-kappaB.

Protection of thrombin receptor expression under hypoxia

Thrombin receptor (ThR) plays a significant role in myocyte contractility and hypertrophy. Heart myocyte ischemic damage, caused by insufficient blood supply, is the leading cause of heart infarction. Here we demonstrate that when primary myocyte cultures are subjected to hypoxic stress, ThR mRNA levels are reduced markedly. This takes place also in vivo in a model of ischemic pig heart, exhibiting reduced levels of ThR compared with normal heart sections. Prior activation of ThR however, by either thrombin receptor-activating peptide (TRAP) or by alpha-thrombin resulted in full protection of ThR mRNA levels under hypoxia. The effect appeared specific to ThR because the addition of TRAP did not affect the hypoxic damage as shown by the levels of lactic dehydrogenase release and up-regulated GLUT-1, a glucose transporter gene. This protection effect took place not only in primary myocytes but also in NIH3T3 fibroblasts. ThR protection occurs via specific cell signaling events because activation of the receptor by TRAP, following interruption of the signaling cascade by calphostin C, a protein kinase C inhibitor, resulted in loss of ThR mRNA protection. Because Ras and Src are part of the ThR signaling cascade, the introduction of either dominant ras or src oncogenes to NIH3T3 murine fibroblasts gave rise to similar protection of ThR mRNA levels under hypoxic conditions without the exogenous addition of TRAP. Likewise, ThR mRNA protection was obtained after transfection with proto-oncogene vav. The 95-kDa protein Vav undergoes tyrosine phosphorylation after ThR activation, serving thus as part of the receptor machinery cascade. We therefore conclude that the initiation of the signaling cascades either exogenously by TRAP or within the cell via src or ras, as well as via vav oncogene interconnecting G-binding protein to the tyrosine kinase pathway, ultimately results in ThR protection under hypoxia. We present hereby, a novel concept of activated receptors, which under minimal oxygen tension protect their otherwise decaying mRNA. Maintaining the level of ThR that plays an active role in normal myocyte function may provide a significant repair mechanism in ischemic tissue, assisting in the regaining of normal myocyte functions.

Activation of protein kinase C is required for the stable attachment of adherent platelets to collagen but is not needed for the initial rapid adhesion under flow conditions

We have investigated the role of protein kinase C (PKC) in the initial events of alpha(2)beta(1)-integrin-mediated platelet adhesion to collagen under flow conditions. Although adhesion caused activation of PKC, as evidenced by pleckstrin phosphorylation, the PKC inhibitors GF 109203X and Gö 6976 had no effect on adhesion, even though they prevented pleckstrin phosphorylation. The initial kinetics and extent of platelet adhesion to collagen (<5 seconds) and tyrosine phosphorylation of p125(FAK) and p72(syk) were not influenced by the PKC inhibitors, whereas adhesion to polylysine was prevented. These results indicate that adhesion to collagen and polylysine involve different mechanisms and requirements for PKC activation. Pretreatment with GF 109203X destabilized collagen-adherent platelets, accelerating their detachment, which was associated with tyrosine dephosphorylation of p125(FAK). Thus, although PKC activation was not required for rapid platelet adhesion to collagen, it appears to play an important role in stabilizing the attachment of adherent platelets to collagen. We also examined the effect of PKC activation by the phorbol ester phorbol 12-myristate 13-acetate (PMA) on platelet adhesion to collagen. PMA at 100 nmol/L strongly potentiated adhesion and tyrosine phosphorylation of p125(FAK) and p72(syk) and activated beta(1)-integrins, as determined by increased exposure of the 15/7 epitope. The PMA-stimulated adhesion was partially blocked by an anti-alpha(2)beta(1) antibody, was completely inhibited by GF 109203X, and was not correlated with the extent of pleckstrin phosphorylation. Therefore, strong PKC activation may lead to inside-out signaling, enhancing the role of beta(1)-integrins in adhesion. Pleckstrin phosphorylation does not appear to be involved in the initial phase of basic or PMA-stimulated adhesion but may help stabilize the adherent platelets.

Integrin alpha(IIb)beta(3) signaling in platelet adhesion and aggregation

Intracellular signals are received and generated by the alpha(IIb)beta(3) integrin on platelets. Recent advances have been made in the areas of agonist receptors that initiate platelet activation, downstream signaling molecules (e.g. small G-proteins and kinases) and changes in ligand-occupied alpha(IIb)beta(3) that cause further signaling and clot retraction.

R-Ras signals through specific integrin alpha cytoplasmic domains to promote migration and invasion of breast epithelial cells

Specificity and modulation of integrin function have important consequences for cellular responses to the extracellular matrix, including differentiation and transformation. The Ras-related GTPase, R-Ras, modulates integrin affinity, but little is known of the signaling pathways and biological functions downstream of R-Ras. Here we show that stable expression of activated R-Ras or the closely related TC21 (R-Ras 2) induced integrin-mediated migration and invasion of breast epithelial cells through collagen and disrupted differentiation into tubule structures, whereas dominant negative R-Ras had opposite effects. These results imply novel roles for R-Ras and TC21 in promoting a transformed phenotype and in the basal migration and polarization of these cells. Importantly, R-Ras induced an increase in cellular adhesion and migration on collagen but not fibronectin, suggesting that R-Ras signals to specific integrins. This was further supported by experiments in which R-Ras enhanced the migration of cells expressing integrin chimeras containing the alpha2, but not the alpha5, cytoplasmic domain. In addition, a transdominant inhibition previously noted only between integrin beta cytoplasmic domains was observed for the alpha2 cytoplasmic domain; alpha2beta1-mediated migration was inhibited by the expression of excess alpha2 but not alpha5 cytoplasmic domain-containing chimeras, suggesting the existence of limiting factors that bind the integrin alpha subunit. Using pharmacological inhibitors, we found that R-Ras induced migration on collagen through a combination of phosphatidylinositol 3-kinase and protein kinase C, but not MAPK, which is distinct from the other Ras family members, Rac, Cdc42, and N- and K-Ras. Thus, R-Ras communicates with specific integrin alpha cytoplasmic domains through a unique combination of signaling pathways to promote cell migration and invasion.

Intracellular expression and functional properties of an anti-p21Ras scFv derived from a rat hybridoma containing specific lambda and irrelevant kappa light chains

A rat single-chain Fv (Y238 scFv) was derived from the Y13-238 monoclonal antibody, a non-neutralizing anti-Ras antibody. The Y13-238 hybridoma expresses two functional light chains. N-terminus microsequencing of these chains showed the presence of the Y3 Ag1.2.3 Vkappa chain derived from the rat fusion partner and of a rat Vlambda chain. Primers designed for rat Vlambda amplification allowed the cloning of a functional scFv that could bind p21Ras. The kinetics of interaction of purified Y238 scFv with the p21Ras protein was evaluated by BIAcore with a NTA sensor chip and gave an apparent affinity constant in the nanomolar range (K(D)=4.58+/-0.63 nM). Immunoprecipitation experiments of Y238 scFv expressed in Xenopus laevis oocytes confirmed the specificity of the scFv for the Ras protein. Y238 scFv could be intracellularly expressed in oocytes and in mammaliam cells without adverse effect on the Ras signalling cascade. This scFv was therefore used as control in experiments where another anti-Ras scFv (Y259 scFv, derived from the neutralizing anti-Ras mAb Y13-259) blocked the Ras pathway in vitro and led to tumor regression in a nude mouse model [Cochet, O., Kenigsberg, M., Delumeau, I., Virone-Oddos, A., Multon, M.C., Fridman, W.H., Schweighoffer, F., Teillaud, J.L., Tocqué, B., 1998. Intracellular expression of an antibody fragment-neutralizing p21 ras promotes tumor regression. Cancer Res. 58, 1170-1176.]. Finally, BIAcore analyses indicated that the epitopes recognized by Y238 and Y259 scFvs are not overlapping and allowed a more precise definition of the Y13-238 epitope.

The F2-isoprostane 8-epiprostaglandin F2alpha increases platelet adhesion and reduces the antiadhesive and antiaggregatory effects of NO

F2-isoprostanes are prostaglandin (PG) isomers produced in vivo through free radical-catalyzed peroxidation of arachidonic acid, which may affect platelet function. The current study investigated the effects of 8-epiprostaglandin F2alpha (8-epi-PGF2alpha) on critical events of platelet activation. A dose-dependent increase in platelet adhesion to fibrinogen- and plasma-coated microwells by 8-epi-PGF2alpha (1 to 1000 nmol/L) was observed when resting platelets (plasma from 1.3+/-0.2% to 5.5+/-0.2%, EC50 of 48 nmol/L; fibrinogen from 3.3+/-0.3% to 6.4+/-0.2%, EC50 of 35 nmol/L; mean+/-SEM, n=8, P<0.001) and thrombin-stimulated human platelets were used. The expression of the adhesion molecule glycoprotein IIb/IIIa was increased by 10 to 1000 nmol/L 8-epi-PGF2alpha in resting platelets (from 64.8+/-2.1% to 83.9+/-1.3%; n=5, P<0.01) and in stimulated platelets. The secretion of the glycoprotein GMP-140 increased only in the presence of both thrombin and 10 to 1000 nmol/L 8-epi-PGF2alpha (from 48.5+/-3.1% to 63.1+/-2.0%, P<0.05). The antiaggregatory effects of both the NO donor NOR-3 (basal, 21.4+/-4.6%; with 8-epi-PGF2alpha, 30.8+/-6.9%; n=14, P<0.05) and endothelial cells that release NO (basal, 18.5+/-4.6%; with 8-epi-PGF2alpha, 30.7+/-5.3%; n=15, P<0.001) were also reduced. All of these effects were prevented by the thromboxane receptor antagonist GR32191 but not affected by acetylsalicylic acid. An increase in free intracellular calcium concentration, measured with the use of fura 2, was observed with 8-epi-PGF2alpha. In conclusion, F2-isoprostanes may participate in oxidative injury by inducing platelet activation and by reducing the antiplatelet activity of NO: increased platelet adhesiveness and expression of the fibrinogen receptor are induced by nanomolar amounts of 8-epi-PG-F2alpha. Platelet secretion and aggregation can also be induced in the presence of platelet agonists.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Integrins and GTPases in tumour cell growth, motility and invasion

One of the most exciting aspects of recent research on cell-adhesion receptors is the realization that these molecules can participate in bidirectional signal-transduction processes. Integrins can convey signals from the extracellular matrix to the cell interior, and intracellular events can influence the affinity of integrins for their ligands. Another important emerging concept is that integrins and small GTPases of the Rho family work together to coordinate both cytoskeletal organization and signalling processes. In this review, the authors examine integrin signalling and integrin-GTPase interplay in the context of cancer cell growth and behaviour.

Tyrosine phosphorylation and relocation of SHIP are integrin-mediated in thrombin-stimulated human blood platelets

The SH2 domain-containing inositol 5-phosphatase, SHIP, known to dephosphorylate inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate has recently been shown to be expressed in a variety of hemopoietic cells. This 145-kDa protein is induced to associate with Shc by multiple cytokines and may play an important role in the negative regulation of immunocompetent cells mediated by FcgammaRIIB receptor. We report here that SHIP is present in human blood platelets and may be involved in platelet activation evoked by thrombin. Platelet SHIP was identified by Western blotting as a single 145-kDa protein. Both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4, 5-tetrakisphosphate 5-phosphatase activities could be demonstrated in anti-SHIP immunoprecipitates of platelet lysate. Thrombin stimulation induced a tyrosine phosphorylation of SHIP, this effect being prevented if platelets were not shaken or if RGD-containing peptides were present, indicating an aggregation-dependent, integrin-mediated event. Moreover, although the intrinsic phosphatase activity of SHIP did not appear to be significantly increased, tyrosine-phosphorylated SHIP was relocated to the actin cytoskeleton upon activation in an aggregation- and integrin engagement-dependent manner. Finally, the striking correlation observed between phosphatidylinositol 3,4-bisphosphate production and the tyrosine phosphorylation of SHIP, as well as its relocation to the cytoskeleton upon thrombin stimulation, suggest a role for SHIP in the aggregation-dependent and GpIIb-IIIa-mediated accumulation of this important phosphoinositide.

Affinity modulation of platelet integrin alphaIIbbeta3 by beta3-endonexin, a selective binding partner of the beta3 integrin cytoplasmic tail

Platelet agonists increase the affinity state of integrin alphaIIbbeta3, a prerequisite for fibrinogen binding and platelet aggregation. This process may be triggered by a regulatory molecule(s) that binds to the integrin cytoplasmic tails, causing a structural change in the receptor. beta3-Endonexin is a novel 111-amino acid protein that binds selectively to the beta3 tail. Since beta3-endonexin is present in platelets, we asked whether it can affect alphaIIbbeta3 function. When beta3-endonexin was fused to green fluorescent protein (GFP) and transfected into CHO cells, it was found in both the cytoplasm and the nucleus and could be detected on Western blots of cell lysates. PAC1, a fibrinogen-mimetic mAb, was used to monitor alphaIIbbeta3 affinity state in transfected cells by flow cytometry. Cells transfected with GFP and alphaIIbbeta3 bound little or no PAC1. However, those transfected with GFP/beta3-endonexin and alphaIIbbeta3 bound PAC1 specifically in an energy-dependent fashion, and they underwent fibrinogen-dependent aggregation. GFP/beta3-endonexin did not affect levels of surface expression of alphaIIbbeta3 nor did it modulate the affinity of an alphaIIbbeta3 mutant that is defective in binding to beta3-endonexin. Affinity modulation of alphaIIbbeta3 by GFP/beta3-endonexin was inhibited by coexpression of either a monomeric beta3 cytoplasmic tail chimera or an activated form of H-Ras. These results demonstrate that beta3-endonexin can modulate the affinity state of alphaIIbbeta3 in a manner that is structurally specific and subject to metabolic regulation. By analogy, the adhesive function of platelets may be regulated by such protein-protein interactions at the level of the cytoplasmic tails of alphaIIbbeta3.